Advantages of Decorrugation of Aeromagnetic Data using the Naudy-Fuller Space Domain Filter Saad Mogren (King Saud University, Saudi Arabia) and Derek Fairhead (University of Leeds, England) Kegs Symposium at Exploration 07

What is De-corrugation? Microlevelling or decorrugation are processing methods to remove line-to-line leveling errors of aeromagnetic data which are visible as linear anomalies parallel to the flight lines, So Corrugation is a low amplitude element of flight line noise still remaining in the aeromagnetic survey data after tie line leveling. These residual errors show significant streaking or corrugations when the grid is visualized using shaded relief methods. If uncorrected then the computation of derivatives becomes dominated by these line-orientated noise effects.

Sources of Corrugation Some sources of corrugations are not very clear line to line differences in flying heights (most important). inaccuracy in the measured positions of crossover points of acquisition and tie lines. If an error arises here, it may introduce an error covering a region that extends one line spacing in each direction. The dimensions of this region would therefore be twice the tie line separation by twice the acquisition line separation. inadequate compensation for the magnetic field resulting from the aircraft. Continue---

Sources of Corrugation If the diurnal fluctuations are not accurately measured at the base station, thus affecting part of the measurements along an acquisition line And finally when adjacent flight lines are in opposite flight directions. Although the contractor of the aeromagnetic in this test area performed standard compensation and diurnal corrections and leveling, the rugged terrain and widely spaced tie lines prevented complete removal of flight line noise as seen in next figures.

The test area: is located in the Arabian Shield, which is a complex Precambrian basement made up of many individual terranes and deformed structures that have highly variable susceptibilities, which make leveling the aeromagnetic flightlines more difficult as any small changes in the flight height would cause remarkable change in the reading, the test area is rugged terrain, the aeromagnetic survey is very old survey , with flight line spacing of 800 m, altitude of 150 m above ground level and flight line direction N. 45° E. Figure 1 Topography of the test area in Arabian Shield superimposed by the aeromagnetic survey flightlines.

Figure 2 Corrugation appeared clearly in the aeromagnetic data. All grids ware shaded with inclination 45˚ and declination 135˚  Microlevelling using Bi-directional line gridding.  FFT frequency domain procedures which is based on a directional cosine filter and Butterworth filters.  Combination of Naudy non-linear filter and the Fuller hanned band pass convolution filter. Tested Microlevelling Methods

Microlevelling using Bi-directional line Gridding There are many methods for microlevelling of dataset collected along survey lines, and the success of any method is largely data- dependant. Microlevelling using the Bi-directional line Gridding, can be applied to gridded dataset alone, however for optimum results it was applied to the original line data. In brief, the procedure involves a grid of leveling errors derived from a magnetic grid. These data are then subtracted from the original line data if available, or from the grid. The Bi- directional microlevelling method obtains the error grid by assuming that the gridded data consist of geology, a regional field, and leveling errors, these are separated out at various stages by low pass filtering during the gridding process. Figure 3 Aeromagnetic data Decorrugated using Bi-Directional Gridding.

Leveling using Bi- directional line gridding involves rotating the aeromagnetic survey lines to E-W or N-S directions. For aeromagnetic data of this study, decorrugation by Bi- directional line gridding did not show perfect results (Figure 3 & 4) probably due to the specification of aeromagnetic surveys as it was mentioned previously that decorrugation is largely data dependent. Figure 4 First vertical derivative (FVD) of the Bi-directional decorrugation. (FVD) was applied on the decorrugated gridded data seen in Figures 3 for closer inspection.

FFT Decorrugation It is a Fast Fourier Transform technique employing a high-pass Butterworth filter in conjunction with a directional cosine filter in the frequency domain. The technique can retain anomalies, from gridded data, in the flight line direction only. These anomalies are further filtered to remove geologically significant signal and to generate a correction grid. The corrections are subtracted from the original data. Figure 5 Aeromagnetic decorrugated By FFT high-pass (Butterworth filter and a directional cosine filter in the frequency domain).

Figure 6 FVD of FFT decorrugation  This method demonstrates a reasonable outcome (Figure 5 &6 ), although in some areas there were some remaining obvious corrugations in this method it is difficult to distinguish between leveling errors and true geological anomalies of similar wavelength parallel to the flight lines. Therefore, the amount of corrugations removed were kept to a minimum by setting the Butterworth filter to four times the line spacing so that only frequencies on the order of the flight line spacing can be passed, and the directional cosine filter to pass wavelength only in the direction of the flight lines.

Decorrugation by Naudy-Fuller filters This decorrugation method utilizes two types of filter: a high pass and low pass filters. The high pass filter is applied perpendicular to the acquisition lines direction, whereas the low pass filter is applied in the acquisition line direction. The aeromagnetic data have been decorrugated by applying the Naudy filter (Dreyer and Naudy, 1968) first followed by a Fuller filter (Fraser et al., 1966) for the high pass filter, and smoothed Fuller for the low pass filter as it will remove any introduced high frequency noise. Decorrugation by this method (Figure 7 & 8 ) removes almost all corrugations and produces well leveled grids. Figure 7 Aeromagnetic data after applaying decorrugation by Naudy-Fuller filters

These correction do not remove geophysical information related to the geology since they extract residual errors with: the longest possible wavelength along the flight lines, the shortest wavelength perpendicular to the lines and the smallest dynamic range. In this study, the filter parameters applied a width twice the flight line spacing and length of at least twice the tie line spacing. Figure 8 FVD of decorrugation by Naudy-Fuller filters

Closer inspection of the methods. First vertical derivative (FVD) was applied on the decorrugated gridded data resulted of the three methods for closer inspection, which confirms that the best results were obtained by using a combination of Naudy and Fuller filters (Figure 8). This method removed (minimized) almost all the corrugation errors, whilst retaining the geological induced magnetic anomalies allowing the maximum resolution of the data to be retained. FVD of the Bi-directional decorrugation FVD of FFT decorrugationFVD of the Naudy-Fuller decorrugation